The present invention relates to processes for forming protective diffusion aluminide coatings. More particularly, this invention relates to a process for controlling the thickness and aluminum content of a diffusion aluminide coating by controlling aluminum activity during the coating process.
The operating environment within a gas turbine engine is both thermally and chemically hostile. Significant advances in high temperature capabilities have been achieved through the development of iron, nickel and cobalt-base superalloys and the use of oxidation-resistant environmental coatings capable of protecting superalloys from oxidation, hot corrosion, etc.
Diffusion aluminide coatings have particularly found widespread use for superalloy components of gas turbine engines. These coatings are generally formed by such methods as diffusing aluminum deposited by chemical vapor deposition (CVD) or slurry coating, or by a diffusion process such as pack cementation, above-pack, or vapor (gas) phase deposition. As depicted in FIG. 1, a diffusion aluminide coating 12 generally has two distinct zones, the outermost of which is an additive layer 16 containing an environmentally-resistant intermetallic represented by MAl, where M is iron, nickel or cobalt, depending on the substrate material. The MAl intermetallic is the result of deposited aluminum and an outward diffusion of iron, nickel or cobalt from the substrate 10. Beneath the additive layer 16 is a diffusion zone 14 comprising various intermetallic and metastable phases that form during the coating reaction as a result of diffusional gradients and changes in elemental solubility in the local region of the substrate 10. During high temperature exposure in air, the additive layer 16 forms a protective aluminum oxide (alumina) scale or layer (not shown) that inhibits oxidation of the diffusion coating 12 and the underlying substrate 10.
Diffusion processes generally entail reacting the surface of a component with an aluminum-containing gas composition. In pack cementation processes, the aluminum-containing gas composition is produced by heating a powder mixture of an aluminum-containing source (donor) material, a carrier (activator) such as an ammonium or alkali metal halide, and an inert filler such as calcined alumina. The ingredients of the powder mixture are mixed and then packed and pressed around the component to be treated, after which the component and powder mixture are heated to a temperature sufficient to vaporize and react the activator with the source material to form the volatile aluminum halide, which then reacts at the surface of the component to form the diffusion aluminide coating. Aluminum-containing source materials for vapor phase deposition processes can be aluminum alloy particles or an aluminum halide. If the source material is an aluminum halide, a separate activator is not required. The source material is placed out of contact with the surface to be aluminized. As with pack cementation, vapor phase aluminizing (VPA) is performed at a temperature at which the aluminum halide will react at the surface of the component to form a diffusion aluminide coating.
The rate at which a diffusion aluminide coating develops on a substrate is dependent on the aluminum activity of the process. Generally, CVD processes and those VPA processes where the source material is in the form of particles (e.g., chunks or pellets) with a fixed alloy composition have a constant aluminum activity as long as source material and sufficient activator are available. In contrast, aluminum activity decreases uncontrollably with time during pack cementation and other above-pack processes as a result of the reduced availability of the aluminum source material and/or activator over time. As a further complication, the rate at which the activator is depleted in pack cementation and above-pack processes increases with process-related anomalies, such as extended preheat times often required to heat the large mass of powder. As a result, aluminide coating thickness can be better controlled with VPA and CVD processes than pack cementation and above-pack processes. However, the aluminum content of the coating cannot be controlled to any certain degree during coating with existing aluminizing process.
In order to control both the final thickness and aluminum content of an aluminide coating, control of the aluminum activity during the diffusion process would be necessary. One advantage of being able to control coating thickness and aluminum content would be the ability to minimize coating growth and maximize environmental resistance by providing an outward-type aluminide coating characterized by a relatively low aluminum content near the original substrate surface, but a high aluminum content at the coating surface. Another advantage would be during the coating of a repaired component with both uncoated (e.g., new repair metal) and coated (e.g., unrepaired) surfaces, where additional coating on the original coating could lead to excessive coating thickness. However, as noted above, the prior art lacks a method by which aluminum activity can be accurately controlled during a diffusion aluminide coating process.
The present invention generally provides a process for forming a diffusion aluminide coating on a substrate, such as a component for a gas turbine engine. The process generally entails placing the substrate in a suitable coating chamber, flowing an inert or reducing gas into and through the coating chamber, and then aluminizing the substrate using an aluminizing technique with a substantially constant aluminum activity, such as a vapor phase deposition process. During the aluminizing process, the amount of unreacted aluminum within the coating chamber is controlled by altering the flow rate of the gas through the coating chamber so that a portion of the unreacted aluminum is swept from the coating chamber by the gas. As a result, greater amounts of unreacted aluminum are swept from the coating chamber with higher gas flow rates, reducing the aluminizing rate and aluminum content of the resulting aluminide coating. In contrast, lower flow rates sweep lesser amounts of unreacted aluminum from the coating chamber, allowing for increased aluminizing rates and higher aluminum contents within the aluminide coating being formed.
In view of the above, the process of this invention is able to produce a diffusion aluminide coating whose thickness can be accurately controlled, as well as its aluminum content through the coating thickness. As a result, a diffusion aluminide coating can be produced whose thickness and aluminum content can be tailored for a particular application. For example, a coating can be produced to exhibit a reduced rate of coating growth and improved environmental resistance as a result of being initially deposited to be an outward-type aluminide coating characterized by a relatively low aluminum content near the original substrate surface, and with an increasing aluminum content toward the coating surface.
Also possible with this invention is the ability to rejuvenate a diffusion aluminide coating on a component, such as a component that has undergone surface repairs. In such circumstances, the aluminum activity is adjusted to such a range that diffusion of aluminum into the substrate is the rate controlling step for coating growth. On those uncoated (e.g., repaired) surfaces of the component, the driving force for coating growth is significant since the aluminum level is controlled by alloy chemistry. For those surfaces of the component that retain the original aluminide coating, the driving force for aluminum deposition is a function of the aluminum level remaining in the coating. Areas with significant aluminum levels in the existing coating will have minimal coating growth, while areas with existing coating that is depleted in aluminum will increase in aluminum content with slight coating growth. As a result, excessive coating thickness is avoided on those surfaces with the original coating.
In addition to the above advantages, the present invention provides a cost effective aluminizing method that is compatible with existing aluminizing processes and equipment. In addition, the method can be selectively implemented without interfering with the process flow of other components that do not require the benefits of this invention.
Other objects and advantages of this invention will be better appreciated from the following detailed description.